This invention relates to a blood pressure monitoring device according to the prior-art portion of claim 1.
Blood pressure monitoring devices of this type include integrated in a housing a display device for indicating the blood pressure measurement values, a control device for controlling the individual components of the blood pressure monitoring device, a power source, a pumping device, and a valve for controlled air inflation and deflation in the bladder of a cuff, a pressure sensor for detecting a pressure signal, an evaluating device for evaluating the pressure signal, and an inclination detecting device preferably arranged in the interior of a housing of the blood pressure monitoring device for detecting the inclination of the blood pressure monitoring device relative to the horizontal and for delivering an electrical inclination signal indicative of the inclination, and, disposed on the housing, an applicator unit for applying the pressure sensor against a subject""s limb in the wrist area of his or her lower arm.
Blood pressure monitoring at a subject""s wrist or finger frequently suffer from lack of measurement accuracy and insufficient repeatability. For one part, this may be attributable to the high sensitivity of the measurements to variations in the measuring position, that is, the individual position of the wrist or finger relative to the position of the heart. In the event of a measuring position deviating from heart level, the measurement result is corrupted by about 0.8 mm Hg/mm due to the hydrostatic pressure differential between the heart and the measuring position. An improper position during a measurement cycle hence produces a systematic measurement error.
Various proposals have been made in the past to obtain improved blood pressure readings in the light of these problems. In particular, proposals have been made to detect the inclination of the lower arm relative to the horizontal because this inclination, given a predetermined position of the elbow as, for example, a position in which it rests against the upper part of the body, is a measure of the level of the wrist and hence of the hydrostatic component of the blood pressure in the wrist area.
An inclination-sensing blood pressure monitoring device is described, for example, in DE 296 12 412 U1. Arranged in the interior of the housing of the blood pressure monitoring device is a disk-shaped pendulum whose circumference visible through a window in the housing is color-marked. This marking indicates an inclination range within which the blood pressure measurement values can be considered as sufficiently correct because the arm is held so that the wrist is positioned roughly at the level of the heart. Similar simple mechanical solutions are also described in Japanese Offenlegungsschrift JP-8-580 (Application Serial No. 6-145168) or in Japanese publication 09038055 A (Application Serial No. 07196590). The requirement for direct observability of the pendulum through the window disposed adjacent to the blood pressure value indicating device results in considerable overall sizes of such blood pressure monitoring devices. Furthermore, handling is rendered difficult because of the need for the pendulum to settle first before reliable readings can be obtained.
The art knows of blood pressure monitoring devices equipped with inclination detecting devices which are in a position to produce an electrical inclination signal which, also for reasons of greater ease of the further processing of electrical signals, is advantageous. Such a blood pressure monitoring device is disclosed, for example, in U.S. Pat. No. 5,778,879 which does not however provide any specifics as to the mode of operation of the inclination detecting device. Japanese Offenlegungsschrift 7-143970 (Application Serial No. 5-295062) describes a blood pressure monitoring device which uses as inclination sensor an electrolyte sensor, not described in greater detail, whose output voltage is varied in response to the inclination by variation of the resistance value. Such sensors require elaborate sealing and should operate on A.C. voltage to avoid changes in the composition of the electrolyte, which again necessitates a substantial control effort.
From U.S. Pat. No. 5,042,505 an electronic sensing device is known in which the change of an angular position relative to a first angular position of the human spinal column can be measured. To this effect, provision is made for an optical encoder with a movable pendulum unit whose degree of rotation up to 360xc2x0 is determinable in a digital electrical signal in the form of electrical pulses per angular degree of rotation of the pendulum. This known inclination detecting device thus comprises a movable positioning element and an inclination sensing device cooperating with the positioning element, which includes at least one sensing element movable with the positioning element and at least one further sensing element, said sensing elements being configured such that the electrical inclination signal is derivable from the relative position of the relatively movable sensing elements.
From SU 1825091 an inclination sensor for measuring rails is known. The inclination measurement supplies a relative signal formed by the position of a pendulum with inductive measuring unit.
From U.S. Pat. No. 5,574,442 an inclination sensor with a movable electrode engaging between two fixed electrodes is known. The movable electrode produces an electrical capacitance between the two other electrodes which varies with the angle of inclination.
A blood pressure monitoring device of the type initially referred to is known from DE 197 57 974 A1.
It is an object of the present invention to provide a blood pressure monitoring device of the type indicated in the prior art portion, which is equipped with an inclination detecting device for delivering an electrical inclination signal, said detecting device being of straightforward construction and operating accurately and reliably.
To accomplish this object, the present invention proposes a blood pressure monitoring device incorporating the features of claim 1.
In blood pressure monitoring devices of the invention, the inclination detecting device comprises at least one movable positioning element and an inclination sensing device cooperating with the positioning element and including at least one first sensing element movable with the positioning element and at least one second sensing element movable relative to the first sensing element, said sensing elements being configured such that the electrical inclination signal is derivable from the relative position of the relatively movable sensing elements. In this arrangement, the positioning element is understood to be a movable member which, in relation to the gravity vector, that is, in relation to the vertical, attempts to occupy invariably the same position of equilibrium, with the inclination of the blood pressure monitoring device being a measure of the inclination of the lower arm. The positioning element may be formed, for example, of a liquid surface or a body floating on a liquid, whose alignment is dictated by the alignment of the liquid surface. The possibility also exists to configure the positioning element as a pendulum movable in one or several axes, that is, as a mass which is, for example, fixedly mounted on a housing outside its center of gravity. Positioning elements rolling off along curved surfaces or operating according to the gyroscopic principle may also be contemplated. The use of the relative movability of the sensing elements and the unambiguous relationship between inclination angle and equilibrium rest position afford the possibility of employing a plurality of preferably physical effects for generation of the inclination signal, utilizing in particular electric, electromagnetic and/or magnetic fields and/or electromagnetic and/or acoustic waves, as well as changes in capacitance resulting from position changes of capacitor surfaces.
The sensing element movable with the positioning unit can be coupled, for example, mechanically or through a field of force, to the movement of the positioning element. Preferably, this sensing element is fixedly connected with the positioning element and/or formed by part of the positioning element as, for example, a surface section thereof. The other sensing element can be fixedly connected with a housing of the blood pressure monitoring device or a mounting structure fixedly attached to the housing, as a card or printed circuit board carrying electronic components or a housing of the inclination detecting device. The relative motion then results from the movement of one sensing element with the blood pressure monitoring device, while the sensing element operatively coupled to the positioning unit maintains or attempts to occupy a position of equilibrium relative to the gravity vector. This arrangement in which the equilibrium relative position of the sensing elements is dictated by the angle of inclination of the blood pressure monitoring device or the subject""s limb carrying it, can be utilized for generation of the inclination signal.
In a further aspect, at least one sensing element is formed by an arc section which is preferably curved uniformly or in the manner of a circular arc. The arc section can be essentially two-dimensional or three-dimensional in the form of a body. The other sensing element is arranged in the area of the arc section, either making contact therewith or being spaced therefrom in close proximity thereto. Depending on the position of the blood pressure monitoring device, a clearly position-related part or portion of the arc section is then operatively associated with the other sensing element so that the inclination signal is obtainable, for example, from a preferably continuous property change of the arc section in the direction of the arc. For example, a surface property of the arc section, as its surface-area-specific or integral reflecting power for optical radiation or light, can be subject to variation in the direction of the arc, and/or at least one dimension of the arc section, for example, its width, can be subject to variation in the arc direction. It is also possible to derive an inclination signal from an inclination-anglede-pendent variation of the distance between the sensing elements as by configuring, for example, the arc section in such manner that its distance from the other sensing element is greater or smaller, depending on the angle of inclination.
In a preferred further aspect, one sensing element, in particular a sensor-active area or a sensor area effective in cooperation with the other sensing element, has a width varying in the direction of relative motion of the sensing elements or in the direction of the arc. This variation proceeds preferably linearly at least in sections, hence enabling an inclination signal to be produced which varies linearly with the inclination angle and lends itself to particular simple evaluation. Conveniently, a wedge or trapezoidal shape of the sensing element is provided for this purpose.
It is of particular advantage for the variation of a property of the arc section related to the direction of the arc or the direction of relative motion to proceed symmetrically relative to a reference position or zero position which corresponds, for example, to a horizontal alignment of the blood pressure monitoring device. In this manner no special evaluating effort is needed to produce an absolute signal for the inclination, and the blood pressure monitoring device can be used equally on either side of a patient""s body. Where deemed necessary, further sensor arrangements may be provided to distinguish between upward and downward inclinations.
The concept of relatively movable, cooperating sensing elements can be made use of for producing the inclination signal in a variety of ways. In another aspect, the positioning element is of a pendulum-type configuration and mounted for rotation about an axis normally fixedly mounted on the housing, and one of the sensing elements is formed by an arc section arranged essentially concentrically with the axis of rotation, particularly by a peripheral section of the positioning element. The other sensing element is arranged preferably immovably in the area of the arc section. One of the advantages of using a peripheral section for forming a sensing element is that already at small angles of adjustment about the axis of rotation relatively large arc lengths can be utilized for signal generation, whereby the measurement accuracy can be enhanced. In yet another aspect, an arc section serving as sensing element is configured as a roll-off surface fixed to the housing and cooperates with another sensing element provided on a rolling member, said rolling member forming the positioning element. Roll-off contact between the sensing elements enables an easy, low-friction adjustment of the equilibrium of the positioning element in spite of relative contact of the sensing elements or the parts carrying them.
To produce the electrical inclination signal a variety of different effective relationships between the sensing elements can be utilized. In one embodiment the inclination sensing device is configured as an optically reflective inclination sensing device in which preferably one sensing element includes at least one reflecting surface for optical radiation while the other sensing element includes at least one source of radiation and at least one radiation detector, said reflecting surface being aligned relative to the radiation source and the radiation detector such that optical radiation from the radiation source which is reflected by the reflecting surface is incident on the radiation detector. For inclination-angle-dependent signal generation, for example, the surface-area-specific degree of reflection and/or the extension of the reflecting surface in the area of the radiation source and/or the radiation detector can vary in the direction of relative motion. Advantageously, the radiation source equipped, for example, with at least one light-emitting diode and the radiation detector equipped with at least one phototransistor or one photodiode may be integrated into a common component in the manner of a reflected light barrier. A preferred embodiment of this type will be explained later as the description proceeds.
It is also possible to configure the inclination sensing device as an optically transmissive inclination sensing device. This may equally include at least one radiation source or radiation emitter and at least one radiation detector or radiation receiver, which operate in a preferably similar region of the spectrum and are associated with one of the sensing elements. The other sensing element, particularly the one which is movable in response to gravity, may include an optically transmissive medium which exhibits a suitable absorption characteristic in this region of the spectrum and may be formed, for example, by a section of an arc o r circumference of a pendulum or the like. The radiation source can radiate into the radiation receiver via an optical path either directly or through a reflector, with the geometrical arrangement being such that the radiation passes through the optically transmissive medium at least once on its path between the radiation source and the radiation detector. In this arrangement the distance covered within the optically transmissive medium can be dependent on the angle of inclination, so that the radiation detector receives a greater or lesser amount of radiation depending on the inclination angle. Also with an optically transmissive measurement it is possible to unite the functions of the radiation emitter and radiation receiver in a single component, for example, in a reflected light barrier adapted to be arranged on one side of a pendulum or the like, or a bifurcated light barrier in which transmitter and receiver can be disposed on opposite sides of the transmissive medium. The possibility also exists to direct the radiation in such manner that it is passed through the optically transmissive medium several times whereby an improved signal-to-noise ratio is obtainable under circumstances.
Preferably, the optical devices referred to in the foregoing are encapsulated in a manner essentially impervious to radiation, thereby preventing the measurement from being affected by stray light or spurious radiation. When provision is made for a reflector in the radiation path between the radiation source and the radiation detector for deviation, this reflector may be formed by an appropriate surface section of an in particular optically radiation-impervious housing surrounding the arrangement. In cases where the housings or enclosures are not impervious to radiation, provision is made for a device for the compensation of residual light. This device enables the same radiation sources and radiation detectors to be employed, with the residual light compensation taking place by (electronic/optical) subtraction of reflections at two different radiation intensities.
It is also possible to utilize the transmission and/or reflection of waves for implementing an acoustically operating inclination sensing device operating with ultrasound, for example.
The optically or acoustically operating inclination sensing devices described are the preferred approach, among other reasons also because the cooperative relationship between the sensing elements can take place in non-contacting fashion and free from reaction forces. As far as forces are concerned, this enables the gravity-induced adjustment of the equilibrium of the positioning element to proceed in a manner completely unaffected by the processes used for measurement of the inclination angle. Furthermore, sealing problems as they occur with electrolyte sensing devices can be eliminated. The devices have a practically unlimited service life and are able to operate largely without wear.
The inclination sensing device may also be configured as a capacitive inclination sensing device in which the sensing elements may act as cooperating charge carrier surfaces. The inclination signal is obtainable from an inclination-angle-dependent variation of the surface dimension of the capacitor formed by the sensing elements and/or from a variation of the relative distance of the cooperating charge carrier surfaces. In one embodiment of a capacitive inclination sensing device the positioning element is configured as a rolling member movable preferably in one axis and designed to roll off along a preferably uniaxially curved rolling surface of the inclination sensing device, wherein the inclination signal occurs in dependence upon the position of the rolling member relative to the rolling surface. At least one charge carrier surface of the inclination sensing device can be associated with the rolling surface, with preferably the width of the charge carrier surface transverse to the direction of motion or direction of rolling of the rolling member varying preferably linearly, for which purpose the charge carrier surface may be of a wedge-type or trapezoidal configuration, for example. An advantageous embodiment of this type is described in connection with the embodiments.
The possibility also exists to provide an inductively operating inclination sensing device in which, for example, one sensing element, in particular a part of the positioning element, is configured as an element guiding the magnetic flux. The other sensing element may include at least one electric coil which preferably surrounds a core made of a magnetizable material for guiding and amplifying the magnetic flux. Together with the element guiding the magnetic flux, the coil arrangement may form an essentially closed magnetic circuit, wherein the magnetic resistance of the magnetic circuit may be dependent on the inclination angle. The sensing elements may cooperate in non-contacting fashion, which is accomplished by an air gap maintained between the coil arrangement curved, for example, in a horseshoe-shaped or circular-arc shaped configuration, meaning its core and the element guiding the magnetic flux. The electromagnetic interaction between the sensing elements can be utilized to advantage in the manner of an eddy-current brake to effect damping of the normally oscillating motion of the positioning element as it moves into its rest position.
The generation of electrical inclination signals possible according to the invention affords a plurality of advantageous possibilities of further processing of the inclination signal, which result in optimal maneuverability of the device and an enhanced measurement accuracy in the determination of blood pressure. The movable positioning element provided in the inclination detecting device which is configured in the manner of a pendulum or as a rolling member, for example, has a characteristic oscillatory response characterizable by a natural frequency. Depending on the way the patient moves, adoption of the measuring position involves exciting the positioning element into oscillating with a more or less high initial amplitude. This oscillation dies down gradually until adjustment of the equilibrium, which may require a prolonged period of time when the oscillation is undamped. Damping of the oscillation of the positioning element is possible actively by means of oil damping, for example. Proceeding from a computed or experimentally determined oscillatory response of the oscillation system comprising the positioning element, the generation of an electrical inclination signal advantageously enables the determination of an estimated value for the position of equilibrium of the positioning element, and hence for the gravity vector, electronically or by computation from a damped inclination signal varying periodically upon a change in inclination, also when the position of equilibrium has not been adopted as yet. In contrast to mechanical solutions, there is hence no need to wait with the reading until the positioning element has died down. For determination of the estimated value, an adaptive electronic filtering of the inclination signal varying periodically during dying-out can be performed.
Depending on the purpose for which the inclination signal is to be used it is also possible to resort to the speed or acceleration of the positioning element, which is possible, for example, by generating the first or second time derivative of the inclination signal electronically or by computation. On the basis of the motion or acceleration of the measurement device or the subject""s limb thereby derivable, it can be established whether the subject is generally at sufficient ease to be able to perform a meaningful measurement. If an increased degree of motor activity is detected so that the blood pressure reading is possibly not representative, the measurement can be aborted and/or a visual or audible warning of the inaccurate reading can be given. Dynamic errors, also referred to as motion artifacts which are attributable to tremor or an uncontrolled arm movement, are thereby avoidable or at least determinable.
The electrical inclination signal also affords an easy possibility of correcting a measured blood pressure value in accordance with the detected inclination and delivering a value which corresponds to a measurement made at the level of the heart. A preferred feature is the provision of a user guidance explained by way of example in connection with the embodiments, which is intended to guide the subject""s arm to an appropriate position prior to the blood pressure measurement cycle. This enables the electronic evaluating device to be of particularly straightforward construction, in addition to achieving a training effect for the user. Furthermore, an advantageous arrangement and/or configuration of a display device described in connection with the embodiments, which is essentially readable only when the subject""s wrist is held against the upper body part at about the level of the heart, serves to enhance the reliability and repeatability of the blood pressure measurements. Motion artifacts can be reduced and the user can be made aware of the level of his or her heart.
These and further features will become apparent not only from the claims but also from the description and the accompanying drawings, and it will be understood that the individual features, whether taken alone or combined in the form of sub-combinations, may be implemented in an embodiment of the invention and in other fields and represent advantageous embodiments.